This invention relates to a device for measuring the mass flow of liquid gas which is pumped in a transfer conduit from a first storage vessel (such as a tank car) through a measuring station to a consumer such as a second storage vessel or the like. At least the transfer conduit portion up to and including the measuring station is cooled prior to measurement, by circulating the liquid gas through the measuring station and a bypass conduit which is arranged downstream of the measuring station and which leads back to the first storage vessel. The temperature to which the conduit portion is cooled is below the boiling temperature of the liquid gas. Upon reaching this desired low temperature, the bypass conduit is shut off, the conduit leading to the consumer is opened and the flow metering starts.
The handling of liquid gases is rendered difficult basically by the fact that at ambient temperature they generate a vapor pressure which lies significantly above the atmospheric pressure and thus these liquid gases tend to pass into the vapor phase. Therefore, they must be maintained at very low temperatures, such as -30.degree. C in case of carbon dioxide and -180.degree. C in case of nitrogen.
From the commercial point of view the most significant type of measuring is the quantity measurement during the course of filling various stationary consumer storage vessels from tank cars. Conventionally, the pumped liquid gas quantity is determined by weighing the consumer vessel before and after the filling operation. This measuring method, however, involves substantial expense, because for each consumer vessel a separate scale has to be made available and the scales have to be calibrated at predetermined periods.
For these reasons more recently measuring processes have been developed which operate on the earlier-described principle and wherein the measuring station contains a propeller (impeller) counter or a turbine counter for effecting continuous flow rate measuring. These counters, however, must not be exposed to the liquid gas flow at the beginning of the pumping operation, because the liquid gas is, in a large measure, in the vapor phase due to the still warm connecting conduits, resulting in significantly higher flow velocities. Consequently, the counters designed for the liquid flow measurement would be overdriven. In order to prevent such an occurrence, it is known to bypass the counter during the pump start operation and by first circulating the liquid gas in a closed circuit for the purpose of cooling the conduits and particularly the measuring station to such an extent that no vapor generation can take place. Only then is the liquid gas caused to flow to the consumer through the measuring station.
A significant difficulty encountered in a system operating as outlined above is the low temperature-caused high degree of bearing wear at the propeller shaft or turbine shaft. Consequently, particularly expensive bearing metals have to be used. When liquid nitrogen is handled, however, even the use of such bearing metals has not led to a satisfactory operational safety or life expectancy.